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It was not perhaps the most romantic of locations to renew their political vows but today, in a railway depot in Birmingham, David Cameron and Nick Clegg did just that.

Insisting, once again, that they, their parties and the country were better off with them together than apart.

The prime minister rejected my suggestion that they resembled a warring couple who've tried to reassure people they were staying together for the sake of the children.

Today was all about emphasising what they do agree on - what David Cameron calls their shared purpose, mission and agenda. First and foremost it is economic but embraces the reform of health and education too.

They refused, though, to answer hard questions about what they do not agree on - in particular, how to resolve the row about reforming the House of Lords which has led the Lib Dems to make increasingly uncoded threats.

If the Tories don't deliver, they warn, they'll retaliate by refusing to vote to shrink the size of the Commons - a reduction in the number of MPs which would, it's estimated, make it easier for the Conservatives to win an election on their own.

It is that which is leading some on both sides to talk about divorce.